As the informatization of society has progressed, problems involving leakage of confidential information have become more serious, increasing demand for the development of a technique of preventing information leakage. For digital data, for example, techniques of encrypting data in order to prevent the content of the data from being looked at by a third party even when the third party has obtained the data have actually been developed and used as an effective countermeasure against information leakage.
However, no technique of preventing leakage of information of printed material such as paper media has been developed sufficiently, and therefore there has been no success in the practical use of such a technique. About fifty percent of information leakage are said to be made through printed material, and accordingly not only a technique of preventing information leakage through digital data but also the development of a technique of preventing information leakage through printed material is considered a pressing urgency.
Specific examples for which countermeasures against information leakage through printed material are desired include bills for product purchases, account statements for credit cards and the like, medical records, school grade reports, rosters, and the like. The present invention can be applied as a technique of encrypting, for example, the important parts of the above forms of information in order to prevent information leakage.
Japanese Laid-open Patent Publication No. 8-179689 (Patent Document 1) is an example of a disclosure of a technique of encrypting information on printed material. In the technique disclosed in Patent Document 1, an entire image is first divided into plural blocks, the images in the blocks are rearranged on the basis of parameters obtained from the input password (encryption key), and a black-and-white inversion process and a mirror inversion process are performed on the images in the blocks specified by the parameters in order to encrypt the image. When the encrypted image is to be decrypted, a positioning frame is set on a portion outside of the image, the password (decryption key) is input, and the original image is restored by executing in reverse order the steps executed for the encryption.
As another example of a conventional technique, there is a technique in which binary data in the form of images is embedded in printed material as disclosed in Japanese Patent No. 2938338 (Patent Document 2). In this conventional technique, binary data is converted into images by expressing binary data in the form of black or white rectangles and arranging them in a matrix. Further, the printed material has positioning symbols set at specified positions on the matrix in order to indicate the position of the embedded image for the decryption. Reference to these positioning symbols makes it possible to photograph the image with a digital camera or the like and decrypt the embedded information.